Secondary recovery process

ABSTRACT

Hydrocarbons are recovered from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well by displacing hydrocarbons toward the production well using a drive fluid such as water thickened with a polymer comprising repeating units of vinyl sulfonic acid alkoxylated with ethylene oxide or a mixture of ethylene oxide and propylene oxide. Optionally, the drive fluid can be saturated with carbon dioxide and/or natural gas at the injection pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well wherein a drive fluid such as water thickenedwith a polymer comprising repeating units of vinyl sulfonic acidalkoxylated with ethylene oxide or a mixture of ethylene oxide andpropylene oxide is utilized to displace hydrocarbons in the formationtoward a production well.

2. Prior Art

The production of petroleum products is usually accomplished by drillinginto a hydrocarbon-bearing formation and utilizing one of the well-knownrecovery methods for the recovery of hydrocarbons. However, it isrecognized that these primary recovery techniques may recover only aminor portion of the petroleum products present in the formationparticularly when applied to reservoirs of viscous crudes. Even the useof improved recovery practices involving heating, miscible flooding,water flooding and steam processing may still leave up to 70-80 percentof the original hydrocarbons in place.

Thus, many large reserves of petroleum fluids from which only smallrecoveries have been realized by present commercial recovery methods,are yet to reach a potential recovery approaching their estimatedoil-in-place.

Water flooding is one of the more widely practiced secondary recoverymethods. A successful water flood may result in recovery of 30-50percent of the original hydrocarbons left in place. However, generallythe application of water flooding to many crudes results in much lowerrecoveries.

The newer development in recovery methods for heavy crudes is the use ofsteam injection which has been applied in several modifications,including the "push-pull" technique and through-put methods, and hasresulted in significant recoveries in some areas. Crude recovery of thisprocess is enhanced through the beneficial effects of the drasticviscosity reduction that accompanies an increase in temperature. Thisreduction in viscosity facilitates the production of hydrocarbons sinceit improves their mobility, i.e., it increases their ability to flow.

However, the application of these secondary recovery techniques todepleted formations may leave major quantities of oil-in-place, sincethe crude is tightly bound to the sand particles of the formation, thatis, the sorptive capacity of the sand for the crude is great. Inaddition, interfacial tension between the immiscible phases results inentrapping crude in the pores, thereby reducing recovery. Anotherdisadvantage is the tendency of the aqueous drive fluid to finger, sinceits viscosity is considerably less than that of the crude, therebyreducing the efficiency of the processes. Another disadvantage is thetendency of the aqueous drive fluid to remove additional gas bydiffusion from the in-place oil thus further reducing the alreadylowered formation oil volume and increasing the viscosity of the oil.

There is a definite need in the art for a water flooding process inwhich the disadvantages discussed above are largely eliminated oravoided.

SUMMARY OF THE INVENTION

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises:

(A) injecting into the formation via an injection well a drive fluidcomprising water having dissolved therein a small amount of a polymercomprising repeating units of vinyl sulfonic acid alkoxylated withethylene oxide, or a mixture of ethylene oxide and propylene oxidewherein the weight percent of ethylene oxide in the said mixture is fromabout 60 to about 95,

(B) forcing the said fluid through the formation and

(C) recovering hydrocarbons through the production well.

An additional embodiment of this invention relates to the drive fluidcompositions utilized in step (a) which may be saturated with carbondioxide and/or natural gas.

DETAILED DESCRIPTION OF THE INVENTION

Prior to practicing the process of this invention it is sometimesdesirable to open up a communication path through the formation by ahydraulic fracturing operation. Hydraulic fracturing is a well-knowntechnique for extablishing a communication path between an injectionwell and a production well. Fracturing is usually accomplished byforcing a liquid such as water, oil or any other suitable hydrocarbonfraction into the formation at pressure of from about 300 to about 3000psig which are sufficient to rupture the formation and to open upchannels therein. By use of this method it is possible to position thefracture at any desired vertical location with respect to the bottom ofthe oilfilled zone. It is not essential that the fracture planes behorizontally oriented, although it is, of course, preferably that theybe. After the fracture has been established, and without diminishing thefracture pressure, a propping agent may be injected into the fraction inorder to prevent healing of the fracture which would destroy itsusefulness for fluid flow communication purposes. Gravel, metal shot,glass beads, sand, etc. and mixtures thereof are generally employed aspropping agents. When sand is utilized as the propping agent particleshaving a Tyler mesh size of from about 8 to about 40 are preferred(i.e., from about 0.016 to about0.093 inches).

The water-soluble polymers utilized in the drive fluids of thisinvention comprise repeating units of alkoxylated vinyl sulfonic acid.Generally, the number average molecular weight of the alkoxylated vinylsulfonic acid polymers will range from about 10,000 to about 2,000,000or more. The polymer of alkoxylated vinyl sulfonic acid can be preparedby the usual vinyl compound polymerization methods at temperatures ofabout 30° to about 100° C. employing a suitable polymerization catalystsuch as azo-bis(isobutyronitrile), ammonium persulfate, etc. Thepreparation of vinyl polymers of this type is described in detail inNorton et al U.S. Pat. No. 3,779,917 and in DePaolo U.S. Pat. No.3,405,003 which are incorporated herein in their entirety by reference.

The alkoxylated vinyl sulfonic acid monomers employed in preparing thepolymers of alkoxylated vinyl sulfonic acid useful in this inventioncomprise vinyl sulfonic acid alkoxylated with from about 2 to about 100percent by weight of ethylene oxide or with a mixture of ethylene oxideand propylene oxide and wherein the weight percent of ethylene oxide inthe said mixture is about 60 to about 95.

The alkoxylation of vinyl sulfonic acid can be conveniently conductedusing methods well known in the art. For example, an aqueous solution ofthe vinyl sulfonic acid comprising about 10 to about 30 weight percentor more of the polymer in water along with about 0.5 weight percent ormore of powdered potassium hydroxide or sodium hydroxide is charged toan autoclave and the autoclave and contents heated to a temperature ofabout 125° to about 200° C. after which the required weight of ethyleneoxide or a mixture of ethylene oxide and propylene oxide is pressuredwith nitrogen into the autoclave over a period of 1 to about 3 hours ormore following which the autoclave is allowed to cool to roomtemperature and then vented. The reaction product remaining is thenstripped to remove unreacted materials, etc. to yield the water-soluble,alkoxylated vinyl sulfonic acid.

A number of other methods are set out in the art for conducting suchalkoxylation reactions including those described in U.S. Pat. Nos.2,213,477, 2,233,381; 2,131,142; 3,879,475; 2,174,761; 2,425,845 and3,062,747.

The following example which illustrates the preparation of thealkoxylated vinyl sulfonic acid is to be considered not limitative.

EXAMPLE I

A total of 425 cc of water, 6.5 g. of powdered potassium hydroxide and48 g. of a vinyl sulfonic acid are added to an autoclave which is thenheated to a temperature of 120° C. Ethylene oxide in the amount of 36 g.is added to the autoclave under nitrogen pressure over a 1.15 hourperiod during which time the temperature of the autoclave is maintainedat 120° C. Next, the autoclave and contents are allowed to cool to roomtemperature after which the autoclave is vented. The reaction mixture isthen stripped of volatiles using a nitrogen purge. The resultingwatersoluble product is vinyl sulfonic acid alkoxylated with about 42weight percent of ethylene oxide.

The following example illustrates the preparation of alkoxylated vinylsulfonic acid polymer and is to be considered not limitative.

EXAMPLE II

A reaction flask equipped with a moderate speed stirrer is charged with100 parts by weight of vinyl sulfonic acid alkoxylated with 51 weightpercent of ethylene oxide prepared as described in Example I, 220 partsby weight of ethanol and 0.73 parts by weight ofazo-bis(isobutyronitrile). The reaction mixture is then flushed withcarbon dioxide to remove oxygen following which the polymerization isconducted for 18 hours at 62° C. Next the reaction mixtures is strippedof volatiles using a nitrogen purge yielding the alkoxylated vinylsulfonic acid polymer which exhibits a number average molecular weightof about 173,000.

In the secondary recovery process of this invention, generally theaqueous drive fluid will contain from about 0.01 to about 5.0 weightpercent or more of the alkoxylated vinyl sulfonic acid polymer.Optionally, the aqueous drive fluid may be saturated with carbon dioxideand/or natural gas at the injection pressure which generally will befrom about 300 to about 3000 psig or more.

If desired, the aqueous drive fluid having dissolved therein theabove-described polymeric thickening agent may be made alkaline byaddition of an alkaline agent. The advantageous results achieved withthe aqueous alkaline medium used in the process of this invention arebelieved to be derived from the wettability improving characteristics ofthe alkaline agent.

Useful alkaline agents include compounds selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxides,and the basic salts of the alkali metal or alkaline earth metals whichare capable of hydrolyzing in an aqueous medium to give an alkalinesolution. The concentration of the alkaline agent employed in the drivefluid is generally from about 0.005 to about 0.3 weight percent. Also,alkaline materials such as sodium hypochlorite are highly effective asalkaline agents. Examples of these especially useful alkaline agentsinclude sodium hydroxide, potassium hydroxide, lithium hydroxide,ammonium hydroxide, sodium hypochlorite, potassium hypochlorite, sodiumcarbonate and potassium carbonate.

A wide variety of surfactants such as sulfonates, alkyl polyethoxylatedsulfates, etc. may also be included as a part of the aqueous drive fluidcomposition. Generally about 0.001 to about 1.0 or more weight percentof the surfactant will be included in the drive fluid.

This invention is best understood by reference to the following examplewhich is offered only as an illustrative embodiment of this inventionand is not intended to be limitative.

EXAMPLE III

In a field in which the primary production has already been exhausted,an injection well is completed in the hydrocarbon-bearing formation andperforations are formed between the interval of 6310-6325 feet. Aproduction well is drilled approximately 435 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 6310-6325 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

In the next step water at a temperature of 70° F. saturated with carbondioxide at about 980 psig to which there has been added about 0.16weight percent sodium hydroxide and containing dissolved therein about0.41 weight percent of a polymer consisting of repeating units of vinylsulfonic acid alkoxylated with 39 weight percent of ethylene oxide andhaving a number average molecular weight of about 105,000 is injectedvia the injection well into the formation at a pressure of about 980psig and at the rate of 1 barrel per minute. Injection of the drivefluid is continued at the rate of 1 barrel per minute and at the end of55 days the rate of production of oil is substantially greater than withwater injection alone.

The sodium, potassium and ammonium salts of the above mentionedalkoxylated vinyl sulfonic acid polymers are also useful as thickenersfor the drive fluids employed in the process of this invention.

What is claimed is:
 1. A process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises:(A) injecting into theformation via an injection well a drive fluid comprising water havingdissolved therein about 0.01 to about 5.0 weight percent of awater-soluble polymer consisting of repeating units of vinyl sulfonicacid alkoxylated with about 2 to 100 weight percent of a materialselected from the group consisting of (I) ethylene oxide and (II) amixture of ethylene oxide and propylene oxide, (B) forcing the saidfluid through the formation, and (C) recovering hydrocarbons through theproduction well, and wherein the number average molecular weight of thesaid water-soluble polymer is about 10,000 to about 2,000,000.
 2. Theprocess of claim 1 wherein the said drive fluid contains a surfactant.3. The process of claim 1 wherein the said drive fluid is saturated withcarbon dioxide at a pressure of about 300 to about 3000 psig.
 4. Theprocess of claim 1 wherein the said drive fluid is saturated withnatural gas at a pressure of about 300 to about 3000 psig.
 5. Theprocess of claim 1 wherein the said drive fluid includes about 0.005 to0.3 weight percent of an alkaline agent selected from the groupconsisting of sodium hydroxide, potassium hydroxide and sodiumhypochlorite.
 6. A fluid comprising water having dissolved therein about0.01 to about 5.0 weight percent of a water-soluble polymer consistingof repeating units of vinyl sulfonic acid alkoxylated with about 2 toabout 100 weight percent of a material selected from the groupconsisting of(I) ethylene oxide and (II) a mixture of ethylene oxide andpropylene oxide and wherein in the said mixture the weight percent ofethylene oxide is about 60 to about 95, and wherein the number averagemolecular weight of the said water-soluble polymer is about 10,000 toabout 2,000,000.
 7. The fluid of claim 6 saturated with carbon dioxideat a pressure of about 300 to about 3000 psig.
 8. The fluid of claim 6saturated with natural gas at a pressure of about 300 to about 3000psig.
 9. The fluid of claim 6 wherein about 0.005 to about 0.3 weightpercent of an alkaline agent selected from the group consisting ofsodium hydroxide, potassium hydroxide and sodium hypochlorite isincluded.
 10. The fluid of claim 6 wherein about 0.001 to about 1.0weight percent of a surfactant selected from the group consisting ofalkylaryl sulfonates and alkyl polyethoxylated sulfates is included.